1. Field of the Invention
This invention is directed to a power source for converting an ac line voltage into a high frequency output ac voltage, and more particularly, to a power source which provides a high frequency output ac voltage with a reduced ripple suitable for operating discharge lamps such as fluorescent lamps.
2. Description of the Prior Art
One known type of such power source for the discharge lamps comprises an inverter, a full-wave diode bridge rectifier and a smoothing capacitor interposed therebetween, the smoothing capacitor being responsible for smoothing the pulsating dc voltage from the rectifier output so as to apply a resulting dc voltage with a reduced ripple to the input terminals of the inverter which in turn provides a high frequency ac voltage output for operating a discharge lamp load. However, this necessitates a smoothing capacitor which has a higher capacitance which should be connected across the output ends of the rectifier to therefore act as a reactive load with a corresponding higher capacitive reactance, thus reducing the power factor which is a serious problem and drawback to a power source of the above kind. For improving the power factor, there has been provided another prior art power source which includes a choking coil connected between the like smoothing capacitor and the like rectifier so as to counteract or reduce the higher capacitive reactance of the smoothing capacitor. Such choking coil, however, should be limited to have a high impedance against a low frequency range and is exceptionally bulky and expensive, which disadvantageously increases the size and the component cost of the power source. In view of the above, there has been proposed another prior power source for improving the power factor, such as is shown in FIG. 1, which comprises a rectifier 3 to provide full-wave rectification of an input ac line voltage fed through a filter 2 composed of a filter capacitor 20 and a filter choke 21, an inverter 5 for receiving the dc voltage from the rectifier 3 and providing a high frequency output ac voltage to a discharge lamp load 6, and a smoothing circuit 4' provided therebetween and consisting of a chopper 40 and a smoothing capacitor 41. The chopper 40 is formed by a choking coil or inductor 42, a switching transistor 43, a control circuit 44 for the switching transistor 43, and an isolation diode 45, and is so arranged that when the voltage developed across the smoothing capacitor 41 exceeds the input line voltage the switching transistor 43 is driven to perform on-off operation for improving the power factor. This prior device permits the employment of an inductor with a much reduced inductance and therefore is of smaller size and is less expensive, one disadvantage exists in that chopper 40 additional to the inverter 5 must require the switching transistor 43 as well as the control circuit 44 thereof, and therefore adds the complexity to circuit and results in a corresponding increase in cost.